I followed Dr. Phuonger's method but instead of using Amp Interface Adapter I used the speaker level inputs that comes with the amp, oem sub wires by tapping into the oem amp, grounded to the bolt in the trunk just beneath the rear mirror, gain is about 1/4, bass eq is 0, LP filter is at 100, i have alpine mrp-m500

I followed Dr. Phuonger's method but instead of using Amp Interface Adapter I used the speaker level inputs that comes with the amp, oem sub wires by tapping into the oem amp, grounded to the bolt in the trunk just beneath the rear mirror, gain is about 1/4, bass eq is 0, LP filter is at 100, i have alpine mrp-m500

Have you tried setting the mrp-m500 gain to minimum and adjusting the bass EQ to taste? That could work...

after subs and amp installed, there is hiss w/ about 1/4 volume turn up, i really do not know if it's coming from sub or amp

Hiss is such noise signal comprising high frequency components, your subs normally are not able to play that noise. I assume you made a wrong grounding. This is the case e.g. the amp is grounded onto the (-) bolt of the battery, thus inducing the low frequency noise from the electric generator into your amp.

Hiss is such noise signal comprising high frequency components, your subs normally are not able to play that noise. I assume you made a wrong grounding. This is the case e.g. the amp is grounded onto the (-) bolt of the battery, thus inducing the low frequency noise from the electric generator into your amp.

i used to ground it here (1st pic), new ground location (2nd pic) to shorten the ground cable because i placed my amp on top of the oem amp

I made a long discussion yesterday with my carhifi dealer for possible solutions to hiss problem and further improvement of sound, finally we found replacing the HU is the best choice. He has already installed several Alpine PXE-H650 into system. I have to say I had misconception about that the volume control have to be taken over by the H650 after the sound calibration. It is because the BMW HU has a variable EQ to volume, and the calibration can just act on only one volume stage. I can still adjust the volume of HU but the EQ is no more optimal at any other volume. Even if I add a line driver and DQXS it is the same. The question why I don't replace the HU with alpine CDA-9887 for just 350$ instead spending 800 $ to add line driver + EQ for a moderate sound improvement.
The CDA-9887 has imprint processor fully integrated, has iPod control, USB-Audio, bluetooth telephony, PDC and MF-wheel are all included. Most important is calibration, EQ, XOver all happen at one place, no sonic degrading due to A/D and D/A converting, and no hiss problem any more. With 4x60w of integrated amp I may even just need one 2 channel amp to drive subs. I think I have no reason not to use the CDA-9887. Only one drawback I have to sell both my amps and buy one amp with RCA inputs, because CDA-9887 has only 4V RCA lineouts. Does anyone has a CDA-9887 HU already installed?

is it a good idea to ground 2 amps (aftermarket and oem) at the same location?

there are several common grounding points distributed at different location physically, electrically they are the same. In the picture you see one gounding point where lots of brawn wires are connected together. On the other side of the same postion there is another one, they are the same. You could even use this one for grounding or use the another one on the other side.

Hiss is such noise signal comprising high frequency components, your subs normally are not able to play that noise. I assume you made a wrong grounding. This is the case e.g. the amp is grounded onto the (-) bolt of the battery, thus inducing the low frequency noise from the electric generator into your amp.

let me make sure how you installed the amp. You taped the speaker level signal from the wires running from oem amp speaker output to subs. Since your alpine mrp-m500 provides with speaker level input therefor you don't need hi-low adpater. If all that are correct, then
1. did you cut down the oem speaker wires where you taped the signal from for your mrp? These speaker wires should be connected to your mrp amp output.
----------- ----- pos.1 --------- -----------
| oem amp | -- speaker wires --> | mrp amp | -- speaker wires --> speakers
----------- --------------------- -----------
2. If your installation was correct as described above, disconnect the speaker wires of pos.1 to test whether you still have the noise. If you still have the noise then your mrp amp is bad.

i cut the oem sub wires from oem amp to subs, soldered the short part from oem amp to speaker level inputs, soldered the long part running to the subs to spades and connected it to the outputs on mrp amp
it's exactly like you described above, thank you for taking time to go thru this with me

From everything I can tell this is an install error. If you have minor hiss with the PDX you need a line driver/pre amp, that is pretty self explanatory. But what you are talking about is flat out weird because from what I can tell your subs are the only ones being driven by the amp...???

Post specific pics showing what wires were tapped into, the connections, your ground etc. That would probably help more than anything.

There is also a possibility that you fried somethign in HU from a short, in which case the CDA may be your best bet.

there is still hiss after the speaker inputs wire is disconnected from the alpine amp, so the amp is bad? my question is, if the input is disconnected, then there is no signal going to the alpine amp, then why is it bad?

there is still hiss after the speaker inputs wire is disconnected from the alpine amp, so the amp is bad? my question is, if the input is disconnected, then there is no signal going to the alpine amp, then why is it bad?

Then it is the noise of its own. Normally such aftermarket amps have S/N rate higher than 90dB, even my PDX with 76dB doesn't have any noise if input is disconnected. Above 90dB should be noisefree absolutely. I assume your car engine was off during the test otherwise the electric magnetic interruption could also be a possible noise origin.
So you could send back you amp.

there is still hiss after the speaker inputs wire is disconnected from the alpine amp, so the amp is bad? my question is, if the input is disconnected, then there is no signal going to the alpine amp, then why is it bad?

And this is after setting the amp gain to the absolute minimum, correct?